An encoder is used for identifying the position of a moving body. Along with improved operation accuracy of servomotors and the like, endeavor has been made to improve position identifying accuracy of an encoder. An example of methods for improving position identifying accuracy is a method of identifying a position by using a so-called “accumulation method”.
According to the accumulation method, a position of a moving body is represented by using a plurality of position data having different resolutions. By identifying positions of the moving body sequentially from the position data with low resolution (superordinate data) to the position data with high resolution (subordinate data), position data representing an accurate absolute position of the moving body are generated. According to the accumulation method, for example, by including the absolute position data of the moving body into the position data with the lowest resolution, the absolute position with high resolution can be also identified.
However, according to the accumulation method, if an error (also referred to as a “phase error”) occurs between two or more pieces of position data having different resolutions, the position data obtained by accumulating the two or more pieces of position data may become inaccurate.
In order to generate accurate position data even when such an error occurs, an encoder described in Japanese Patent No. 3551252 or Japanese Patent No. 3336396, for example, has been developed. According to the encoder, despite in a different manner, by using subordinate data and superordinate data in a process of generating position data, a section in the superordinate data in which the subordinate data are obtained is identified to generate the position data representing an absolute position from the section identified and the subordinate data. Therefore, compared with an example in which data are simply accumulated, by compensating the phase error described above when identifying the section, more accurate position data are generated.
According to the encoder described in Japanese Patent No. 3551252 or Japanese Patent No. 3336396, the phase error is corrected in the process of generating position data based on two or more pieces of position data having different resolutions. However, a deviation amount that can be corrected is limited. Although such a limited amount may depend on a correction method or resolutions of various position data, even when the limited amount is the maximum amount, for example, if the position data with lower resolution exceeds a half of a phase of the position data with higher resolution, the phase error can be hardly corrected.
One of the causes of the phase error is, for example, an error included in a detection signal used in generating the position data. The phase errors are broadly classified into non-reproducible errors and reproducible errors in accordance with characteristics of the error included in the detection signal. The non-reproducible error is caused by real-time driving conditions such as transfer of a moving body or vibration. The reproducible error is caused by a detection mechanism itself such as a manufacturing error. Therefore, according to the encoder described in Japanese Patent No. 3551252 or Japanese Patent No. 3336396, for example, if a non-reproducible error depending on real-time driving conditions is accumulated on the reproducible error included in the encoder itself originally, for example, reality is that the limited amount of correction is exceeded.